Investments in Unconsolidated Joint Ventures Narrative (Details) $ in Thousands	Jan. 24, 2019 USD ($) ft²	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net		$ 2,959,908	$ 2,964,572
Annapolis Junction Building Six [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	50.00%
Mortgage notes payable, net	$ 13,000
Debt Instrument, Basis Spread on Variable Rate	2.00%
Number of extensions	1
Extension Option (in years)	1
Debt instrument, borrowing capacity	$ 14,300
Net Rentable Area (in sf) | ft²	119,000